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Photoelectric Switch for complete newb

Discussion in 'General Electronics Discussion' started by Chase Standifer, Mar 12, 2015.

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  1. Chase Standifer

    Chase Standifer

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    Mar 12, 2015
    Let me start off with saying that I am a complete newb when it comes to electronics. I understand the difference between in parallel and in series and I can solder slightly better than a 3 year old with his first set of crayons.

    I have built a display cabinet with some LED light strips running off of a 8AA battery pack. I am wanting to add a photoelectric switch so that the LEDs turn on when the light is turned on in the room. I am assuming it is slightly more complicated than wiring a photoresistor or something into the circuit.

    I saw the recent similar thread but what he wanted to do and the provided circuit was far more complicated than what I need. I would appreciate any help with ideas and the parts I would need to wire this up. Thanks in advance
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Essentially you need the photoresistor to turn on a transistor which will, in turn turn on your LEDs.

    Take a look at the first part of this thread. In this thread they initially want light to turn off the LEDs, but it's a relatively simple change to reverse the behaviour. (in the end the behaviour is more complex, but look at sugestions in the first few posts)

    It will give you an idea of the complexity (I think it's pretty simple)
     
  3. Chase Standifer

    Chase Standifer

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    Mar 12, 2015
    [​IMG]
    I found this circuit on a video by Build Electronic Circuits which does exactly what I want. I have also found the parts used in the video. But will these resistors be correct for running two 16" LED light strips vs. one small LED bulb? Also there are two types of LDRs to order. One is 100mw and the other is 250mw, what is the difference there?

    I believe I understand the way the circuit works but there are 1000000 choices for each of the different components and it has been awhile since high school physics and my history degree is useless here.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It all depends on how the loads need to be driven.

    Can you tell me more about the led strip?

    There is probably little practical difference between those ldrs.

    You may require a transistor capable of driving a heavier load.
     
  5. Chase Standifer

    Chase Standifer

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    Mar 12, 2015
    All I really know about the lights is that they require 12vs. I bought them already wired up to the battery pack so Im not sure of the exact brand but from what I can a tell from my research them seem to average 72watts per roll of 5 meters. I have used .72 meters, so if my math is right that is about 55 watts. What specifically would help?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yep, that's fine.

    I recommend that you use the transistor to switch a relay, and the relay used to switch the 12V to the led strip. The single led can be switched by the transistor.

    You will have to be careful that the less are not shining on the ldr as this may affect the operation of the circuit.

    Also note that the circuit as shown has no hysteresis. However using it to switch a relay may add a little, allowing it to operate without continually switching on and off at certain light levels.
     
  7. Chase Standifer

    Chase Standifer

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    Mar 12, 2015
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Firstly take a look at this resource.

    https://www.electronicspoint.com/resources/using-a-bipolar-transistor-to-turn-a-load-on-and-off.30/

    This will show you how to wire the relay to the transistor. Note especially the need for a diode. In that resource a motor is shown but a relay is connected the same way.

    Your circuit diagram above shows a battery voltage of 9V. If this is true, you need a 9V relay.

    The delay you have selected is also one suited for a very high current and requires a fairly high coil current. If you can find one requiring a lower current it would be a lot better. Oh, I just realised your switching 6A. A 10A relay would be fine but it might not require a significantly lower coil current.

    In any case, you will need to try this circuit out on a small scale first to ensure you're happy with the way it switches. If you're not, a more complex circuit may be required.
     
  9. Chase Standifer

    Chase Standifer

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    Mar 12, 2015
    So I have wired this up and can't seem to figure out why it wont work. When I check the resistance across different parts of the circuit things seem fine except for when measured across the transistor where I get and I get a 1 on the multimeter when I measure the circuit as a whole. I get no luck when I hook the relay even when bypassing the LDR. I would appreciate any help where I may have wired something wrong. As bad as the soldering looks there is nothing soldered together that shouldn't be.

    [​IMG]
     
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